tvscl is a built in IDL function that gdl does not have. idlastro.gsfc.nasa.gov/idl_html_help/TVSCL.html
tvscl has been available in GDL since March 2005:
Please upgrade your GDL version?
If installing from source, please remember to do "make install" since numerous library routines written in GDL (including TVSCL) are then installed into the system together with the GDL binary.
Hope that helps,